Method of fabricating a high Q - large tuning range micro-electro mechanical system (MEMS) varactor for broadband applications

ABSTRACT

A Micro Electro-Mechanical System (MEMS) varactor ( 100, 200 ) having a bottom electrode ( 116 ) formed over a substrate ( 112 ) and a dielectric material ( 130 ) disposed over the bottom electrode ( 116 ). A pull-down electrode ( 122 ) is formed over spacer ( 120 ) and the dielectric material ( 130 ). The MEMS varactor ( 100, 200 ) is adapted to operate in a stiction mode, with at least a portion of pull-down electrode ( 122 ) in contact with dielectric material ( 130 ). The MEMS varactor ( 100, 200 ) has a high Q, large tuning range, and high sensitivity.

TECHNICAL FIELD

[0001] This invention relates generally to integrated circuits, and moreparticularly to Micro Electro-Mechanical System (MEMS) devices.

BACKGROUND OF THE INVENTION

[0002] In the telecommunications industry, the demand for lightweightportable devices such as personal computing devices, Personal DigitalAssistants (PDA's) and cellular phones has driven designers to reducethe size of existing components. A Q value is a ratio of the powerstored in a device to the dissipated power in a device. Due to the needfor Q values beyond the capabilities of conventional IC technologies,board-level passive components continue to occupy a substantial portionof the overall area in transceivers of handheld telecommunicationsequipment, presenting a bottleneck against further miniaturization. Forexample, discrete components currently occupy approximately 50% of thespace in cellular phones.

[0003] Recently MEMS devices including resonators, filters, and switcheshave been developed that offer an alternative set of strategies fortransceiver miniaturization and improvement. MEMS devices are high-Q,chip-level, lower power replacements for board-level components thatgreatly decrease space and area requirements.

[0004] One such MEMS device is an RF switch for switching RF signals,shown in a cross-sectional view in FIG. 1. RF drumhead capacitive MEMSswitch 10, disclosed by Goldsmith et al. in U.S. Pat. No. 5,619,061,comprises an insulator 14 such as SiO₂ deposited over a substrate 12,which may comprise silicon, for example. A bottom electrode 16 is formedon insulator 14 and a dielectric 18 is formed over bottom electrode 16.Capacitor dielectric 18 typically comprises Si₃N₄, Ta₂O₅ or othersuitable dielectric materials, for example. An active element comprisinga thin metallic membrane 22 is suspended away from electrode 16 by aninsulating spacer 20. Membrane 22 which serves as a top electrode ismovable through the application of a DC electrostatic field betweenmembrane 22 and bottom electrode 16. Membrane 22, dielectric 18 andbottom electrode 16 comprise a metal-dielectric-metal capacitor when theMEMS switch 10 is in the “on” position, shown in FIG. 2. In the “off”position shown in FIG. 1, with no voltage applied to membrane 22 andbottom electrode 16, the capitance value is at a minimum. MEMS switches10 have low insertion loss, good isolation, high power handling, andvery low switching and static power requirements.

[0005] A MEMS switch 10 may be designed for use as a varactor. Avaractor is a discrete electronic component, usually comprising a P-Njunction semiconductor, designed for microwave frequencies, in which thecapacitance varies with the applied voltage. Varactors are sometimesreferred to as tunable capacitors. Varactors are used in frequency upand down conversion in cellular phone communication, for example.Existing varactors are usually p-n diodes specifically designed foroperation in the reverse bias regimes where the capacitance(C_(j)) ofthe depletion region is varied to set frequency (

_(o)) of operation as reflected in Equation 1:

_(o)≈1/(C _(J) *R _(S) *R _(P))^(1/2)  Equation 1

[0006] where resistances R_(p) and R_(s), are the parallel and seriesresistances of the diode, respectively. Some primary requirements of avaractor are that it have a high quality factor (Q) for increasedstability to thermal variations and noise spikes, and a large lineartuning range (TR). High-performing varactors are usually made of GaAs.Unfortunately, these devices use a different processing technology thatis not amenable to integration into standard Si-CMOS process.

[0007] MEMS devices offer a means by which high Q large tuning rangevaractors can be integrated in higher level devices such as voltagecontrolled oscillators and synthesizers using the current Si-CMOSprocess. The drumhead capacitive switch 10 shown in FIG. 1 may bedesigned to produce a MEMS varactor. The voltage across the electrodesis varied to pull down and up membrane 22, which varies the distanceD_(air) between membrane 22 and dielectric 18, which changes thecapacitance of the device 10 accordingly.

[0008] A problem in MEMS devices is stiction, which is the unintentionaladhesion of MEMS device 10 surfaces. Stiction may arise from the stronginterfacial adhesion present between contacting crystallinemicrostructure surfaces. The term stiction also has evolved to ofteninclude sticking problems such as contamination, friction drivenadhesion, humidity driven capillary forces on oxide surface, andprocessing errors. Stiction is particularly a problem in current designsof MEMS varactors, due to the membrane 22 possibly adhering todielectric 18, resulting in device 10 failure, either temporarily orpermanently. To prevent stiction, material and physical parameters, andvoltage signal levels of the varactor are designed to avoid contact ofmembrane 22 with dielectric 18. Coatings such as Teflon-like materialsthat resist stiction are frequently applied over dielectric 18.

SUMMARY OF THE INVENTION

[0009] The present invention achieves technical advantages as a MEMSvaractor designed to operate in a stiction mode. The pull-down electrodeor top membrane maintains contact with the underlying dielectriccovering the bottom electrode during operation of the varactor. As thevoltage across the pull-down electrode and the bottom electrode isvaried, the area of the pull-down electrode contacting the dielectric isvaried, which varies the capacitance.

[0010] Disclosed is a MEMS varactor, comprising a bottom electrodeformed over a substrate, a dielectric material disposed over the bottomelectrode, and a spacer proximate the bottom electrode. A pull-downelectrode is disposed over the spacer and the dielectric material,wherein the varactor is adapted to operate in a stiction mode.

[0011] Also disclosed is a method of manufacturing a MEMS varactor,comprising depositing an insulator on a substrate, forming a bottomelectrode on the insulator, and depositing a dielectric material overthe bottom electrode. A spacer is formed over the insulator, and apull-down electrode is formed over the spacer and the dielectricmaterial, wherein the varactor is adapted to operate in a stiction mode.

[0012] Further disclosed is a method of operating a MEMS varactor,comprising applying a voltage across the bottom electrode and thepull-down electrode to produce a predetermined capacitance across thebottom and pull-down electrode, wherein at least a portion of thepull-down electrode is adapted to contact the dielectric material duringoperation in a stiction mode.

[0013] Advantages of the invention include solving the stiction problemsof the prior art by providing a varactor adapted to operate in astiction mode. The present MEMS varactor is a high Q varactor having alarge tuning range. The distance between the dielectric and the membranemay be increased in accordance with the present invention, allowing fora larger tuning range and providing more sensitivity to a change involtage. A wider range of voltages and capacitances is available withthe present MEMS varactor design. Furthermore, the use of Teflon-likecoatings on dielectric to prevent stiction of membrane is not required,as in some prior art designs. A wider variety of dielectric materialsmay be used for dielectric than in the prior art because there is noneed for concern about stiction of the membrane to the dielectric. Theinvention provides an extended tuning range that is not possible withonly an air gap for the capacitive medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above features of the present invention will be more clearlyunderstood from consideration of the following descriptions inconnection with accompanying drawings in which:

[0015]FIG. 1 illustrates a cross-sectional view of a prior art MEMScapacitive RF switch;

[0016]FIG. 2 illustrates a cross-sectional view of the MEMS varactor ofthe present invention adapted to operate in a stiction mode, with themajority of the membrane above the bottom electrode in contact with thedielectric;

[0017]FIG. 3 illustrates a top view of the MEMS varactor shown in FIG.2;

[0018]FIG. 4 shows a model schematic representation of the MEMS varactorhaving a capacitance across the membrane and the bottom electrode;

[0019]FIG. 5 illustrates a capacitance to voltage relationship of theMEMS varactor output capacitance over a range of voltages;

[0020]FIG. 6 illustrates a cross-sectional view of the present MEMSvaractor with a portion of the membrane in contact with the dielectric;

[0021]FIG. 7 illustrates a top view of the MEMS varactor shown in FIG.6; and

[0022]FIG. 8 illustrates a cross-sectional view of the present MEMSvaractor with a minimal portion of the membrane in contact with thedielectric and having an increased spacer height, increasing the tuningrange of the varactor.

[0023] Corresponding numerals and symbols in the different figures referto corresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] A cross-sectional view of the MEMS varactor 100 of the presentinvention is shown in FIG. 2. MEMS varactor 100 comprises an insulator114 deposited over a substrate 112, and a bottom electrode 116 formed oninsulator 114. A dielectric 130 is formed over bottom electrode 116 toeliminate the possibility of electrode/electrode fusion and for creatinga capacitance that is greater than possible with air. Spacer 120 areformed over the insulator 114 for supporting membrane 122 a distance D₁above insulator 114. Distance D₁ may be, for example, 0.5-2.0micrometers. Membrane 122 is also referred to herein as a pull-downelectrode or top electrode. Membrane 122 may comprise holes 124 whichare used to remove a temporary filler material (not shown) from cavity126. Membrane 122 is movable through the application of a DCelectrostatic field across membrane 122 and bottom electrode 116,similar to the operation of the MEMS RF switch 10 previously discussed.

[0025] The MEMS varactor 100 of the present invention is adapted tooperate in a stiction mode. A stiction mode is defined herein as anactive operating mode during which a voltage is applied across membrane122 and bottom electrode 116, and the membrane 122 maintains contactwith at least a portion of dielectric 130 covering bottom electrode 116.

[0026] The amount of area or portion 132 of membrane 122 that contactsdielectric material 130 is varied to change the capacitance C. Thecontact portion 132 is varied by changing voltage V across electrodes122 and 116. In the stiction mode, the maximum capacitance C_(max) isachieved when membrane 122 is biased with a voltage V such that membrane122 makes complete contact at portion 132 to dielectric 130 as shown inFIG. 2. Capacitance C_(max) may be expressed by Equation 2,

C _(max≈∈) _(die)A/D_(die)   Equation 2

[0027] where A is the cross-sectional area 132 of the electrode 122 incontact with dielectric 130, ∈_(die) is the dielectric constant of thedielectric 130 covering bottom electrode 116, and D_(die) is thethickness of the dielectric 130. The capacitance is reduced bydecreasing the membrane 122/dielectric 130 contact area, shown in FIGS.6-8, which is accomplished by changing the voltage V. The relationshipof capacitance C to area A, where A is varied by changing the voltage V,is a linear relationship. The minimum capacitance C_(min), expressed inEquation 3, occurs when the membrane 122 is not contacting thedielectric 130,

1/C _(min≈)1/(∈_(air) A/D _(air))+1/(∈_(die) A/D _(die))  Equation 3

[0028] where ∈_(air) is the dielectric constant of the air and D_(air)is the thickness of the air space between membrane 122 and top ofdielectric 130. The tuning range TR is reflected by Equation 4

TR=(C _(max) −C _(min))/C _(min)×100%  Equation 4

[0029] The tuning range of the MEMS varactor may be extended or reducedby changing the material parameters, e.g. the materials of dielectric130 and distances D_(air) and D_(die), of Equations 2 and 3, forexample.

[0030]FIG. 3 illustrates a top view of the MEMS varactor shown in FIG.2, with a circular region 132 of membrane 122 in contact with dielectric130 in a maximum amount, giving a maximum capacitance value C_(max) forthe varactor 100. FIG. 4 shows a model schematic representation of theMEMS varactor 100 having a capacitance C between the membrane 122 andthe bottom electrode 116 for a voltage signal V input to eitherelectrode 122, 116 of the varactor 100. FIG. 5 illustrates thecapacitance to voltage relationship of the MEMS varactor 100 over arange of voltages, for example, a range of voltage signals from 3 to 10volts produces a capacitance ranging from 13 to 25 pF in the stictionmode. These voltages and capacitances are exemplary and may vary withair gap distances D₁ and dielectric material properties. FIG. 6illustrates a cross-sectional view of the present MEMS varactor with aportion 136 of membrane 122 in contact with dielectric 130, membraneportion 136 being smaller than membrane portion 132 shown in FIG. 2.FIG. 7 illustrates a top view of the MEMS varactor 100 shown in FIG. 2,with circular portion 136 of membrane 122 in contact with dielectric130. FIG. 8 illustrates a cross-sectional view of an alternateembodiment of the present MEMS varactor 200 with a minimal portion 138of membrane 122 in contact with dielectric 130 and having an increasedspacer 120 height D₂, increasing the tuning range of the varactor 200.Increasing the distance D₂ to greater than 2 micrometers also providesmore sensitivity to a change in voltage signal V.

[0031] There are many preferred and alternate configurations for thepresent varactor 100, 200 adapted to operate in a stiction mode. A firstvoltage signal applied across the bottom electrode and the pull-downelectrode produces a first capacitance, and a second voltage signalapplied across the bottom electrode and the pull-down electrode producesa second capacitance, where the first and second voltages are different.

[0032] Although preferably the pull-down electrode 122 maintains contactwith the dielectric material 130 over a range of voltage signals, thevaractor 100, 200 may also be operated in a non-stiction mode in analternate embodiment. In this embodiment, the tuning range of thevaractor may be increased if the membrane starts at the undeformed (novoltage signal applied) position and then is deflected so that it makescontact with bottom electrode. The height of the membrane is varied overthe air gap until it makes contact partially, then fully with the bottomelectrode. In this embodiment, a larger tuning range is achievable.However, the varactor may not be reliably operated across the entiretuning range if the membrane permanently sticks, in which case thevaractor would then operate only in the stiction mode.

[0033] The invention also includes a method of manufacturing a MEMSvaractor 100, 200 comprising depositing an insulator 114 on substrate112, forming bottom electrode 116 on insulator 114 and depositingdielectric material 130 over bottom electrode 116. Spacer 120 are formedover insulator 114, and pull-down electrode 122 is formed over spacer120 and dielectric material 130, wherein the varactor 100, 200 isadapted to operate in a stiction mode. At least a portion 132, 136, 138of pull-down electrode 122 contacts dielectric material 130 in astiction mode.

[0034] The invention also includes a method of operating a MEMS varactor100, 200. The method comprises applying a voltage signal V across bottomelectrode 116 and pull-down electrode 122 to produce a predeterminedcapacitance C across bottom 116 and pull-down 122 electrode, wherein atleast a portion 132, 136, 138 of pull-down electrode 122 is adapted tocontact dielectric material 130 during a stiction mode.

[0035] The novel MEMS varactor 100, 200 of the present inventionachieves technical advantages by providing a high Q varactor having alarge tuning range and increased sensitivity. MEMS varactor 100, 200solves the stiction problems of prior art MEMS varactors by beingadapted to operate in a stiction mode. The distance D₁, D₂ between thedielectric and the membrane may be increased in accordance with thepresent invention, allowing for a larger tuning range. A wider range ofvoltages and capacitances is available with the present MEMS varactordesign compared with the prior art. Furthermore, the use of Teflon-likecoatings on dielectric 130 to prevent stiction of membrane 122 is notrequired as in some prior art designs. A wider variety of dielectricmaterials may be used for dielectric 130 than in the prior art becausethere is no need for concern about stiction of the membrane 122 to thedielectric 130. The invention provides an extended tuning range that isnot possible with only an air gap for the capacitive medium.Furthermore, the MEMS varactor 100, 200 preferably comprises siliconrather than GaAs, and may comprise metals that maintain low insertionloss and good isolation of the MEMS varactor 100, 200.

[0036] While the invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications in combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. For example, although membrane portions 132 and 136in contact with dielectric 130 are shown in a top view as beingcircular, other shapes for contact membrane portion 132, 136 areanticipated, for example, square, oval rectangular, or any othergeometrical shape. The MEMS varactor 100 may be designed to also operatein a non-stiction mode, wherein membrane 122 is not in contact withdielectric 130, as well as the stiction mode described herein. It istherefore intended that the appended claims encompass any suchmodifications or embodiments.

What is claimed is:
 1. A Micro Electro-Mechanical System (MEMS)varactor, comprising: a bottom electrode formed over a substrate; adielectric material disposed over said bottom electrode; a spacerproximate said bottom electrode; and a pull-down electrode over saidspacer and said dielectric material, wherein said varactor is adapted tooperate in a stiction mode.
 2. The MEMS varactor according to claim 1wherein a first voltage signal applied across said bottom electrode andsaid pull-down electrode produces a first capacitance.
 3. The MEMSvaractor according to claim 2 wherein said a second voltage signalapplied across said bottom electrode and said pull-down electrodeproduces a second capacitance.
 4. The MEMS varactor according to claim 3wherein said pull-down electrode maintains contact with said dielectricmateral over a range of voltage signals.
 5. The MEMS varactor accordingto claim 4 wherein said range of voltage signals is from approximately 3V to 10 V, wherein a capacitance in the range of 13 to 25 pF isproduceable by said MEMS varactor in response to said range of voltagesignals.
 6. The MEMS varactor according to claim 4 further comprising aninsulating layer disposed over said substrate beneath said bottomelectrode, wherein a distance D₁ is defined between said insulatinglayer and said pull-down electrode.
 7. The MEMS varactor according toclaim 6 wherein said distance D₁ is approximately 0.5-2.0 micrometers.8. The MEMS varactor according to claim 6 wherein said distance D₁ isgreater than 2.0 micrometers.
 9. A Micro Electro-Mechanical System(MEMS) varactor, comprising: a bottom electrode formed over a substrate;a dielectric material disposed over said bottom electrode; a spacerproximate said bottom electrode; and a pull-down electrode over saidspacer and said dielectric material, wherein said varactor is adapted tooperate in a stiction mode, wherein a voltage signal applied across saidbottom electrode and said pull-down electrode produces a capacitance.10. The MEMS varactor according to claim 9 wherein said pull-downelectrode maintains contact with said dielectric materal over a range ofvoltage signals in said stiction mode.
 11. The MEMS varactor accordingto claim 10 wherein said range of voltage signals is from approximately3 V to 10 V, wherein a capacitance in the range of 13 to 25 pF isproduceable by said MEMS varactor in response to said range of voltagesignals.
 12. The MEMS varactor according to claim 11 further comprisingan insulating layer disposed over said substrate beneath said bottomelectrode, wherein a distance D₁ is defined between said insulatinglayer and said pull-down electrode.
 13. The MEMS varactor according toclaim 12 wherein said distance D₁ is approximately 0.5-2.0 micrometers.14. The MEMS varactor according to claim 12 wherein said distance D₁ isgreater than 2.0 micrometers.
 15. A method of manufacturing a MEMSvaractor, comprising: depositing an insulator on a substrate; forming abottom electrode on said insulator; depositing a dielectric materialover said bottom electrode; forming a spacer over said insulator;forming a pull-down electrode over said spacer and said dielectricmaterial, wherein said varactor is adapted to operate in a stictionmode.
 16. The method according to claim 15 wherein said pull-downelectrode contacts at least a portion of said dielectric material insaid stiction mode.
 17. The method according to claim 15 wherein adistance D₁ is defined between said insulating layer and said pull-downelectrode.
 18. The MEMS varactor according to claim 6 wherein saiddistance D₁ is approximately 0.5-2.0 micrometers.
 19. The MEMS varactoraccording to claim 6 wherein said distance D₁ is greater than 2micrometers.
 20. A method of operating a MEMS varactor having a bottomelectrode formed on a substrate, a dielectric material disposed over thebottom electrode, a spacer formed on the substrate supporting apull-down electrode, wherein a voltage applied across the bottomelectrode and the pull-down electrode responsively changes thecapacitance of the varactor, comprising: applying a voltage signalacross the bottom electrode and the pull-down electrode to produce apredetermined capacitance across said bottom and pull-down electrode,wherein at least a portion of said pull-down electrode is adapted tocontact said dielectric material during a stiction mode.
 21. The methodaccording to claim 20 wherein the area of said pull-down electrodeportion in contact with said dielectric material varies responsively tochanges in said voltage signal.
 22. The method according to claim 21wherein said applying a voltage signal comprises applying a voltage ofapproximately 3 V to 10 V to produce a varactor capacitance in the rangeof 13 to 25 pF.